Make sed command GNU-compatible

doc/update/8.1-to-8.2.md

@@ -90,7 +90,7 @@ Update the GitLab init script and 'default' file.
 cd /home/git/gitlab
 sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
 test -e /etc/default/gitlab && \
-  sudo sed -i .pre-8.2 's/^\([^=]*\)gitlab_git_http_server/\1gitlab_workhorse/' /etc/default/gitlab
+  sudo sed -i.pre-8.2 's/^\([^=]*\)gitlab_git_http_server/\1gitlab_workhorse/' /etc/default/gitlab
 ```
 
 Make sure that you also update your **NGINX configuration** to use